The initial phase of this study began in 2008 with the comparison of an electronic national prescription database and the medication history obtained by the Emergency Department (ED) nurse. The goal was to assess the degree to which data from a national prescription electronic database added to the routine medication history and the percent of ED patients who had such data. We showed that the prescription database, (which could be accessed securely from any hospital or clinic), increased the completeness of the patients' medication history by 28%. However, only two-thirds of the patients' prescription information could be found in the database. These results were published in a clinical journal in emergency medicine. Since then we obtained access to other sources of de-identified prescription and patient outcome information from: 1) Symphony Health Rx, a commercially available de-identified database with medication and encounter (with diagnoses) data. Symphony provided a one year sample containing more than 53 million prescription dispensings for 5.5 million people, with associated encounter information, from the Washington DC metropolitan area; b) from MIMIC II a large de-identified database of Intensive care data which we have also used to assess the independent risk of obesity on 30-day mortality after an ICU admission; and c) most recently we obtained access to the Centers for Medicare and Medicaid Services (CMS) Enclave with access to 6 years of prescription and 12 years of immunization, and 12 years of outcome data. With the additional databases we are obtaining experience with the use of big data to answer useful epidemiologic and clinical questions. Ongoing studies include: epidemiology of drug-drug interactions and the difference in the alerts generated by leading drug-drug interaction knowledgebases, incidence of prescriptions of potentially teratogenic drugs to pregnant women, and the association between influenza /pneumonia events and the timing of influenza vaccines, and association between Simvastatin use and Dementia/Alzheimer.